1. Field of the Invention
The present invention relates to an electric power steering apparatus that applies an assist force generated by a motor to a vehicle steering system.
2. Discussion of the Background
A rack-and-pinion steering apparatus is one type of known steering apparatuses for vehicles. In this type of steering apparatus, a steering wheel is connected to a pinion shaft having pinion teeth, and the pinion shaft rotates as the steering wheel is rotated. The pinion teeth of the pinion shaft mesh with rack teeth of a rack shaft, and the rack shaft moves along its axis as the pinion shaft rotates. The two ends of the rack shaft are connected to two steered wheels (front wheels) by tie rods, respectively. The steering angle of the steered wheels changes as the rack shaft moves in its axial direction.
An electric power steering apparatus including the known rack-and-pinion steering apparatus has an auxiliary motor and a ball screw mechanism for assisting the axial movement of the rack shaft. The ball screw mechanism includes a ball screw nut having a helical ball groove in its inner circumference, a helical ball groove provided in the outer circumference of the rack shaft, and a plurality of balls interposed between the two ball grooves. The ball screw nut and the balls convert the rotation of the motor into axial movement of the rack shaft.
A first prior art of an electric power steering apparatus, which includes a reduction mechanism for transmitting the rotation of a motor to a ball screw nut while reducing the speed of the rotation, is described in Japanese Laid Open Patent Publications Nos. 60-25853 and 8-99643.
In the first prior art, the reduction mechanism includes two spur gears which mesh with each other. One of the spur gears is connected to the rotating shaft of the motor, and the other one of the spur gears is fixed to a ball screw nut of a ball screw mechanism. In order to transmit the power of the rotating shaft of the motor to the ball screw nut via the two spur gears, the motor is arranged parallel to the rack shaft.
A second prior art of a steering apparatus is described in Japanese Laid Open Utility Model Publication No. 6-44674. The steering apparatus includes a reduction mechanism including a worm and a worm wheel that mesh with each other. The worm is connected to a rotating shaft of a motor, and the worm wheel is fixed to a ball screw nut of a ball screw mechanism. In order to transmit the power of the rotating shaft of the motor to the ball screw nut via the worm and the worm wheel, the rotating shaft of the motor is arranged perpendicular to the rack shaft.
A third prior art of an electric power steering is described in Japanese Laid Open Patent Publication No. 2000-95123. The steering apparatus includes a rack shaft, a stator having a coil that covers the rack shaft, and a rotor arranged coaxially with the rack shaft. The rotation of the rotor is converted into the movement of the rack shaft by a ball screw.
In a typical vehicle, other devices such as an engine, a transmission, etc., are arranged around the rack shaft. However, in the first prior art, the rotating shaft of the motor can only be arranged parallel to the rack shaft. In the second prior art, the rotating shaft of the motor can only be arranged perpendicular to the rack shaft. Therefore, the power steering apparatuses of the first and second prior arts have problems in that the motor interferes with the installation of other devices of the vehicle and the freedom of design in determining positions for installing other devices is restricted.
In the third prior art, the rack shaft, the stator, and the rotor are retained in a rack housing. The rack housing, which is relatively large, results in problems in that the rack housing interferes with the installation of other devices of the vehicle (the engine, transmission, etc.) and the amount of freedom of design in determining positions for installing other devices is restricted.
In addition, in the first to third prior arts, fastening means, such as bolts and nuts, integrally connects a casing retaining the electric motor to a housing, which retains a power-assist unit for transmitting the power of the electric motor to a steering gear mechanism. Therefore, there is a problem in that the vibration and noise of the electric motor are transmitted to the steering apparatus through the housing retaining the power assist unit and ultimately transmitted to the driver and the passenger compartment. In order to solve this problem, Japanese Laid Open Patent Publication No. 2000-350397 describes an electric power steering apparatus in which an electric motor and a housing are connected to each other by a bushing. The bushing has a metal body covered slightly by a rubber elastic member. Thus, the electric motor and the housing are connected to each other only by the metal body, and the transmission of vibration and noise of the electric motor cannot be blocked sufficiently. In addition, the electric motor is supported by the elastic member in a floating manner with respect to the housing, and rattling of the electric motor may occur due to the floating support. When the electric motor rattles, there is a risk of the rotating shaft of the electric motor applying excessive force to the power transmission shaft of the power assist unit, and therefore the power assist unit cannot be operated smoothly.